This invention concerns compositions which can be described as melt processable pseudointerpenetrating networks of silicones in thermoplastic matrices. This invention also relates to methods for the formation of these compositions.
Previous investigations have demonstrated that silicones may be incorporated into thermoplastic resins at low levels in order to enhance wear, friction and release properties. These silicones, however, are low molecular weight resins which are readily extractable from the matrix resins. Incorporation of silicone at levels above 2% can cause catastrophic reductions in mechanical properties and melt rheology.
The present invention reveals that judiciously selected silicone systems which are vulcanized within a thermoplastic matrix to form pseudointerpenetrating polymer networks (also referred to as semi-interpenetrating polymer networks or semi-IPNs) will not adversely affect polymer properties.
Interpenetrating polymer networks are described by L. H. Sperling in Interpenetrating Polymer Networks and Related Materials, Plenum Press, New York and London (1981). A full or true interpenetrating polymer network (IPN) is a material containing two polymers, each in network form, with the two polymers having been polymerized or vulcanized independently in the presence of each other to form two networks which are intertangled (interpenetrated) with each other. The IPNs may be formed in different manners, with the synthesis (polymerization) and/or cross-linking (vulcanization) of the two polymers being sequential or simultaneous. Another mode of IPN synthesis involves simultaneous coagulation and cross-linking of two latex polymers to form an interpenetrating elastomeric network.
One type of IPN system is illustrated in U.S. Pat. No. 4,302,553 of Frisch et al. IPNs of this sort involve a blend of two different prepolymers cross-linked in independent processes and permanently entangled with one another. These IPNs are thermoset in character.
Semi- or pseudo-IPNs have only one cross-linked phase or network which is within a continuous unlinked polymer matrix phase. It is possible with certain solvent soluble resins to extract this non-cross-linked phase, whereas that is not possible for the true IPN. As a result, the true IPN systems must be cast since once the components are admixed and the polymer formation takes place, the interpenetrating networks cannot be separated. The single cross-linked network of the pseudo- or semi-IPNs allows these materials to retain thermoplastic character, although pseudo-IPNs with thermosetting properties are also possible.